why it is important for the membrane of a cell, such as a muscle cell, to be selectively permeable.?

The cell must have a system that allows some materials in and some out. Take a muscle cell- it needs fuel and oxygen to function. In order to get the energy used from these two materials is a complex system, because simple ignition would result in destruction of the cell, several other chemicals are used to produce a "slow burn". All of which result in things the cell must purge and replace. These must pass through the membrane- while retaining enough material to still be a cell. If all materials were allowed to pass then the cell could not maintain the balance needed to still be a cell, if none were allowed to pass it could not function. A selective permeable membrane is the solution.

Well, you don't want important molecules that are necessary for the life of the cell to just leave and vice versa, you don't wan harmful molecules of any kind to just enter whenever they are around.